Freeze may ease invasive insect spread

The cold weather brought on by the polar vortices does more than raise home heating bills.

It may kill off some invasive bugs in the forest, including the wooly adelgid, the emerald ash borer and the gypsy moth, according to Rick Turcotte, an entomologist for the U.S. Forest Service.

He said most of the data comes from lab work and field surveys, but “there is a pretty good mortality when there are extremely cold temperatures over a short period of time.”

He added that a long winter can reduce numbers with temperatures that aren’t quite as cold as a brief deep freeze, but the foreign species still have proteins that protect them against the cold through the end of February.

The ash borer is found under the bark of ash trees affording them some protection, at least until the trees freeze.

Historic low temperatures will also affect gypsy moth eggs, which are found in small masses on the surface of tree bark.

“There have been a couple of studies of winter hardiness, mostly north of Pennsylvania for the insects where the over-winter mortality had a significant impact,” he added. “But it would have to kill 90 percent or more to slow population growths.”

“We’ll just have to wait and see. We don’t have a good handle on the winter dynamics,” he concluded.

Pennsylvania Department of Conservation of Natural Resources District Forester Cecile Stelter said,”When I have heard references to cold weather substantially affecting forest pests usually it is in context of very cold temperatures, -20 degrees F or colder that are sustained over a period of time (5 or more nights/days). That is not to say that there might not be some beneficial affects to our recent and upcoming cold temperatures. It might slow the rate of spread of the pests and there might be some mortality to the insects.”

“But as with most things, there is a good and bad to everything. The colder temperatures can also cause breakage and cracking of tree limbs and once the weather warms, these can provide an entry-ways for pathogens. And if it is cold enough to kill the pests, it might also be cold enough to kill some of the parasitoids that have been introduced as part of the control methods for these forest pathogens,” she said.

“It is hoped that this cold weather will be of some help, if not to kill the insects then to slow them down.”

Andrea Hille, the forest silviculturist for the Allegheny National Forest said there might be a silver lining to the cold weather.

“But, the amount of adelgid mortality and the long term implications for adelgid populations and hemlock health are pretty difficult to quantify. We probably have not had enough sustained cold for long enough here locally to really result in significant adelgid mortality, based on observations from other places in the eastern United States,” she said.

“I think most of us familiar with hemlock wooly adelgid and its spread across the eastern United States agree that cold temperatures have helped slow the spread, and consequently the rate of hemlock infestation and mortality,” she added.

Hille said that there is some uncertainty regarding how cold temperatures will affect HWA populations on the Allegheny Plateau. Monitoring of populations in the next few years may provide a better understanding of adelgid populations locally and how winter temperatures may or may not influence them.